Generally, color printing is accomplished by creating monochrome subsets of an image and combining the subsets to obtain the full color image. For example, digital printers typically generate color output by developing each of four electrostatic latent images with a cyan (C), magenta (M), yellow (Y) or black (K) “process color”, then superimposing the developed images to form the complete image. Production printing presses generate color hardcopies by generating several printing plates, covering each plate with a different color ink and transferring the ink from the separate plates to the hardcopy sheet. Most printing presses generate at least four printing plates, one for providing ink in each of the process colors.
Each of the C, M, Y and K colorants absorbs light in a limited spectral region of the range of visible light; cyan colorant absorbs red light, i.e., prevents light having a wavelength of approximately 650 nm from being reflected from the image, magenta colorant absorbs green light (light having a wavelength of approximately 510 nm) and yellow colorant absorbs blue light (light having a wavelength of approximately 475 nm). Black colorant absorbs all wavelengths of light and can be deposited onto the latent image rather than depositing all three colorants at the same location. The printable colors are produced by combining the different colorants in various ratios. For example, to generate a blue region in a hardcopy image, relatively high amounts of colorant will be deposited onto corresponding locations of the C and M separations, with little or no colorant deposited in the corresponding location of the Y separation. The cyan and magenta colorants will absorb the red and green light and thus, only blue light will be reflected from the hardcopy sheet and perceived by the viewer.
While printers can usually reproduce a significant number of colors by combining process colors, such combinations cannot be used to reproduce every color in the spectral range of visible light. Printing presses typically provide the additional colors by transferring specially mixed “spot color” inks from printing plates that are formed for that purpose. Digital printers cannot typically add spot colors and thus, the color gamut for a printer is somewhat limited. The size of the gamut colors for a particular printer depends upon several factors, including the processing capability of its image processor, the type and quality of the marking material and output media used and the viewing environment.
Computer generated color images are printed on both printing presses and color printers. The data used to reproduce these images is sometimes provided in “bit map” form, with color values assigned to the pixels in each separation to independently control the deposit of C, M, Y or K marking materials. Image data is also provided as page descriptions that describe how the printer should mark the page in order to print the document. Printer Control Language, produced by Hewlett Packard, Palo Alto, Calif., Advanced Function Presentation, produced by IBM, Armonk, N.Y. PostScript and Portable Document Format (PDF), both produced by Adobe Systems Incorporated of Mountain View, Calif. are examples of well known page description languages (PDLs) can generate printable image data.
While PDLs generally provide a device-independent way to print images, some aspects of page description are device dependent. For example, PDLs use a look-up stored on the host computer to define the colors of the objects in the image. Generally, a look-up table is a database that associates color names or other references to the color values that control the deposit of marking material in the concentrations required by a specific printing device. As these color values are determined based upon the output characteristics of a specific device, the colors in the printed output are often incorrect if the file is printed using another printer. It would be advantageous to process color image data to provide consistent colors in images that are printed on any device.